Accelerometer having integral fault null

ABSTRACT

An improved accelerometer comprising a transducer responsive to vibration in machinery which produces an electrical signal related to the magnitude and frequency of the vibration; a decoding circuit responsive to the transducer signal which produces a first fault signal, and isolating circuitry responsive to the first fault signal to produce a second fault signal in which ground shift effects are nullified.

This is a division of application Ser. No. 07/834,451, filed Feb. 12,1992 now U.S. Pat. No. 5,309,149.

ORIGIN OF THE INVENTION

The invention described herein was made by an employee of the UnitedStates Government and may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

1. Technical Field

The present invention relates to apparatus and method for monitoringmachine vibration. More particularly, the present invention is directedto an accelerometer package containing a vibration transducer with anintegral and improved frequency detection circuitry having significantadvantages over conventional accelerometers.

2. Background Art

Vibration analysis has been used for years to provide a determination ofthe proper functioning of different types of machinery, includingrotating machinery and rocket engines. A determination of a malfunction,if detected at a relatively early stage in its development, will allowchanges in operating mode or a sequenced shutdown of the machinery priorto a total failure. Such preventative measures result in less extensiveand/or less expensive repairs, and can also prevent a sometimescatastrophic failure of equipment. In a rocket thruster, for instance,certain frequencies can be calculated as a function of the geometry ofthe thruster dimensions and, if detected, indicate thruster instability.If thruster vibration is monitored with respect to these criticalfrequencies or is detected in bands of frequencies around the criticalfrequencies, then it is possible to either change operating modes orprovide an orderly shutdown of the thruster prior to failure. A changeof operating mode for a thruster may include changing control signals tothe thruster from a pulsed mode to a continuous mode signal. If such achange in operating mode is not successful to eliminate vibrations at acritical frequency, then it becomes necessary to shut down the thruster.

Accelerometers which are commercially available are contained within apackage that houses only the transducer itself and may include wiring toa connector. In application, this packaging presents a number ofproblems. The signal wire from the accelerometer will typically gothrough a connector and then make its way through an electrically noisyenvironment before reaching a second connector, which will take thesignal to a detector circuit. Noise may be picked up on this signal wireand cause false machine instability signals. Also, because the detectorcircuitry and accelerometer transducer may be located some distanceapart and may be powered by different power supplies, ground loop errorsmay occur. Ground loops or ground shifts are differences between circuitgrounds which may occur when circuits are powered by different powersupplies, or are powered by the same power supply when the circuits arelocated a considerable distance apart. Relatively higher currentscarried by one part of a grounding system can also result in voltagedifferences between other parts of a-grounding system. These voltagedifferences between grounds, which are often called ground shifts orground loops, are typically variable, unknown, and insidiously causefalse signals. When adding vibration monitoring and detecting circuitryit may be difficult or impossible to determine whether ground loopsexist in other circuitry which may cause errors or false signals in thevibration monitoring apparatus. Similarly, noise and ground loopproblems may occur when connecting an instability or fault signal wirefrom the detection circuitry of a vibration monitoring apparatus to adistant control circuit or computer which utilizes the fault signal tosupervise a machine shut down. Typically, vibrating machinery willproduce a noisy electrical environment. As well, the vibration mayproduce occasional changes in electrical resistance in contacts,connectors, etc. which may produce spurious machinery fault signals.

U.S. Pat. No. 4,977,395 to R. J. Bozeman discloses an adjustable delaycircuit which can be used to delay the output of an instability signaluntil the particular type of vibration signal which denotes instabilityhas occurred consistently enough over a period of time (e.g. 40 msec fora rocket thruster) to indicate an instability signal which is morelikely to be valid. However, adding this delay circuit increases overallcircuit complexity which increases the possibility of a circuit failure,and requires additional room. Also, the output of the delay circuit isitself subject to the noise and ground loop problems so that its purposemay still be defeated even if it is effective to distinguish spurioussignals up to the input of the delay circuit.

Consequently, a need exists for improvements in accelerometer circuitryto provide a highly reliable means for preventing false signals from theaccelerometer transducers and related circuitry operating in avibrating, electrically harsh environment. Those skilled in the art havelong sought and will appreciate the novel features of the presentinvention which Solves these problems.

STATEMENT OF THE INVENTION

The present invention is directed, in a preferred embodiment, to animproved accelerometer package containing not only a transducer but alsothe necessary frequency decoder circuitry used to provide a faultsignal, thereby providing a "smart" accelerometer package. This packageis designed to be powered from a single power source. The resultingpackaged system may be incorporated within an accelerometer case duringthe original manufacturing process, or may be retro-fitted intocommercially available accelerometers. Conventional integrated circuitsand components may be used for this purpose. Alternatively, micrologicand surface mount technology may be employed to significantly reduce thepackage size.

Due to the proximity of both the transducer and decoder circuitry withina single housing powered by a single power source, noise problems andground loop problems typically found between the transducer and itsrelated circuitry are greatly reduced to improve reliability ofoperation. The reduction of noise has an added benefit of reducing thenecessary filtering and amplification of the transducer signal, so thatcomponents which perform these functions can be eliminated. In apreferred embodiment, the present invention allows for adjustment of thewidth of the band of frequencies contained in the transducer signalwhich will cause the decoder circuitry to produce an instability outputsignal. Also, it is possible to select an approximate number of pulsesor cycles within the frequency band which must occur prior to thedecoder circuit producing an instability output signal. This feature isavailable by preferably taking advantage of an inherent trait of thedecoder circuit, and requires no additional circuitry. This pulse orcycle counting feature also is an effective delay which helps preventspurious instability signals. In a preferred embodiment, the faultsignal output is coupled to machinery control or a computer via opticsto preclude false shutdown signals by system ground loops.

Accordingly, it is an object of the present invention to provide anintegrated "smart" accelerometer package which is inherently reliable inoperation in monitoring machine vibrations, regardless of unknown groundloops and electrical noise.

A feature of the present invention is the ability to vary the band widthof frequencies which produce a machine instability signal.

Another feature of the present invention is the use of pulse or cyclecounting to form a delay in production of the fault signal untilsufficient data is received to confirm the validity of any fault signalproduced.

An advantage of an integrated smart accelerometer package is a greatreduction in noise and ground shift error that is associated withmachinery vibration monitoring equipment.

Another advantage of an integrated smart accelerometer package with anoptically isolated output is that it may be connected to otherelectronic equipment that may have a ground that is at a differentvoltage than the ground of the accelerometer package without concern ofspurious signals being generated due to this difference in ground.

These and further objects, features, and advantages of the presentinvention will become apparent from the following detailed description,wherein reference is made to the figures in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of an accelerometer package assembled ontoa thruster in accord with the present invention.

FIG. 2 is an elevational view, in section, of an accelerometer packagein accord with the present invention.

FIG. 3 is a circuit diagram of frequency decoder circuitry in accordwith the present invention.

While the invention will be described in connection with the presentlypreferred embodiment, it will be understood that it is not intended tolimit the invention to this embodiment. On the contrary, it is intendedto cover all alternatives, modifications, and equivalents as may beincluded in the spirit of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to instable machinery vibrationdetection apparatus which includes not only a transducer which may beused to measure machine vibration, but also contains integralelectronics which may be used to determine if a fault condition hasoccurred, thereby creating a "smart" accelerometer package.

An accelerometer package, designated by the numeral 12, according to apreferred embodiment of the present invention is shown in FIG. 2. Anexternal housing 28 holds electronics circuit 18 including circuit board31 and mounted electronic components 27 in close proximity to transducer20. External housing 28 provides shielding from electromagnetic fieldswhich may be in the general vicinity of the package due to radiotransmitters, etc.(not shown). By enclosing all necessary componentswithin external housing 28, the signals from transducer 20 contain lessnoise and have a relatively greater signal to noise ratio. This allowsfor a reduction in filtering and amplification circuity. Six pinconnector 22 provides a ready method for electrically connecting theaccelerometer package 12 to thruster control circuitry 24 (see FIG. 1).

FIG. 1 shows accelerometer package 12 mounted on rocket thruster 10adjacent oxidizer valve 14. Related rocket thruster components includingfuel valve 16 and electrical junction box 34 are also indicated inFIG. 1. Thruster control and warning circuitry 24 uses the fault signalfrom accelerometer package 12 to change operating modes of thruster 24and/or to shut down thruster 10 in an orderly manner before failure.Also thruster control and warning circuitry 24 may activate anindication of failure, such as a warning light or bell (not shown).Thruster control and warning circuity 24 may include actual electroniccircuitry or a software controlled computer. It is highly desirable toavoid any false or spurious indications of machine or thruster failurewhich may result from noise and ground loops. A ground loop, or voltagedifference, may exist between ground 17 and ground 15 due to a varietyof reasons, some of which may be unknown and difficult to detect duringoperation. These ground loops may cause a spurious indication ofthruster failure and may be difficult to detect during operation ofrocket thruster 10. A combination of transducer 20 and electronicscircuit 18 into the same housing 28 greatly reduces the possibility ofspurious signals due to ground loops and other noise which otherwisecould detrimentally affect transducer 20 and related circuitry 18.Additional means for preventing spurious signals due to typicallytransient differences in voltage between grounds 17 and 15 are discussedsubsequently.

FIG. 3 provides a preferred embodiment of circuit 18 contained withinaccelerometer package 12. A single power source (not shown) is connectedat connector 22, pin 1, with a common ground shown at pin 2. This sourcevoltage is filtered by capacitor 21 and is used by regulator 40 toprovide 5 volts at output pin O. The regulated 5 volt output is filteredby capacitor 25. Field effect diode 23 regulates current flow totransducer 20. Transducer 20 changes resistance in response to "G"forces, thus creating an electrical transducer signal which correspondsin magnitude and frequency to machine or thruster vibrations. "G" forcesare generally understood to be the forces which act on a mass describedby using the force of gravity as a unit force acting on a mass, e.g.,two "G"s would be a force acting on a mass twice that which would occurdue to gravity. Frequency adjust 38 is set for a desired centerfrequency selected to be equal to the expected unstable vibrationfrequency of the machinery or thruster to be monitored and may becalibrated by reading with a frequency counter (not shown) at pin 6 ofdecoder 32. A frequency band around the desired center frequency may beset using switch 28 which selects either wide bandwidth capacitor 26 foran approximately 300 hertz bandwidth or narrow bandwidth capacitor 29for a bandwidth of approximately 75 hertz. With narrow band widthcapacitor 29 connected to switch 28, approximately 50 incoming cyclesignals of a requisite magnitude within the frequency band are requiredbefore a fault condition is indicated at pin 8 of decoder 32. With widebandwidth capacitor 26 connected to switch 28, approximately 10 to 15cycles are required to generate a fault signal. These values arecalculated assuming a center frequency of 1000 Hz, and will typicallyvary with a change in center frequency. The pulse counting feature whichis inherent to preferred embodiment LM567 decoder circuit 32 preventstransient pulses within the window from generating a fault signal. Bytaking advantage of this inherent feature of the LM567 decoder circuit32 which might in other circumstances be a disadvantageouscharacteristic, it is possible to obtain an effective delay in thecircuitry without adding extra delay circuitry. The LM567 decodercircuit 32 includes a loop filter connection at pin 2 so that capacitor26 or 29 is used to complete this loop filter. The LM567 decoder circuit32 may also use external resistive components (not shown). A generaldescription of the LM567 decoder is disclosed on pages 5-51 through 5-55of the publication entitled "Semiconductor Master Selection Guide 1989"by National Semiconductor and is incorporated herein by reference.

In operation, alternating current from transducer 20, that correspondsin magnitude and frequency to machine vibration "G" forces, passesthrough signal capacitor 27 to signal threshold adjust 30 whichdetermines the desired amplitude trip level based on accelerometercalibration data. For example, if a basic accelerometer produces 1 voltper "G" force and the amplitude threshold desired is 1.5 "G's", asimulated signal at the desired frequency and at 1.5 volts (typicallyapplied with a signal generator) is fed into circuit 18, and signalthreshold adjust 30 is used to set a trip level which produces an outputat pin 8 of decoder 32 or pins 3 and 4 of connector 22. All adjustmentsdescribed above might be set during accelerometermanufacture/calibration, and the entire package permanently encapsulatedwith a sealing material to minimize or eliminate effects of vibrationduring use. Alternatively, these calibration sequences may be determinedby the user if field adjustments are desired.

A machinery fault condition results in the impedance seen at pin 8 ofdecoder 32 changing from a high impedance to a low impedance due to thesinking of an open collector integral to decoder 32. As a result,current flows through light emitting diode 35 between pins 1 and 2 ofoptical isolator 38. A resulting output then occurs at pins 3 and ofoptical isolator 38, which is also available at pins 3 and 4 ofconnector 22. A separate power supply, load, and ground connection topins 3 and 4 of connector 22 is made through indicated wires 46, 47, and48 which results in a circuit independent of the common circuit groundwire or land 42 connected to pin 2 of connector 22. Therefore, if returnwire 46 is analogous to ground 15 of FIG. 1, if ground 42 of FIG. 3 isanalogous to ground 17 of FIG. 1., and if grounds 15 and 17 of FIG. 1are at different voltages with respect to each other, the output signalreceived from accelerometer package 12 by thruster control 24 will beunaffected by this voltage difference. The accelerometer package 12 andthruster control 24 circuits are thus electrically isolated from eachother via optical coupling. The optical isolator 38 precludesunwarranted or inadvertent shutdown caused by electrical ground loopconditions of which an end user of accelerometer package 12 may not beaware, since the output of optical isolator 38 is independent of theground at pin 2 of connector 22. For backup and corroboration purposes,the output of transducer 20 is also made available at pin 5 of connector22 through direct current blocking capacitor 33.

The foregoing description of the invention has been directed in primarypart to a particular, preferred embodiment in accordance with therequirements of the patent statutes and for the purposes ofillustration. It will be apparent, however, to those skilled in the artthat many modifications and changes in the specifically describedaccelerometer package 12 may be made without departing from the scopeand spirit of the invention. For example, although developed foraccelerometers, the teachings of this invention could be used withvarious types of transducers generating dynamic signals of userinterest, i.e., pressure transducers, acoustic sensors, flowmeters, etc.Micro-circuitry and surface mount technology may allow control circuitryor parallel decoders operable at different frequencies to be includedwithin accelerometer package 12. Other means for isolating the output ofthe package may also be used. For example, it may be possible in somesituations to use differential transmission of the fault signal fromaccelerometer package in accordance with RS-422 or RS-485 as defined bythe Electronics Industry Association (EIA) so that the effects of groundshifts and noise signals will appear as common mode voltages on thetransmission line and thereby be nullified. If it is desired to have alatched output condition upon fault detection rather than a pulse, anoptical silicon controlled rectifier (SCR) may be used, and could bereset by an external reset switch or by dropping the load. It may bedesirable for some applications to have an output pulse of a fixed timeduration so that circuitry may be added to produce a pulse having afixed time duration in response to a variable width fault pulse signal.Therefore, the invention is not restricted to the preferred embodimentillustrated, but covers all modifications which may fall within thescope of the spirit of the invention.

What is claimed is:
 1. A machinery vibration monitoring apparatus,comprising:a transducer responsive to vibration in machinery operativeto produce an electrical transducer signal functionally related to themagnitude and frequency of said vibration; a decoding circuit responsiveto said electrical transducer signal greater than a predeterminedmagnitude within a selected frequency band to produce a first faultsignal after a predetermined approximate number of cycles have occurredof said electrical transducer signal within said frequency band; andisolating circuitry responsive to said first fault signal to produce asecond fault signal in which ground shift effects are nullified.
 2. Theapparatus of claim 1, wherein:said isolating circuitry includes opticalisolation circuitry.
 3. The apparatus of claim 1, further comprising:ameans for adjusting the width of said frequency band.
 4. The apparatusof claim 1, further comprising:a means for adjusting said predeterminedapproximate number of cycles of said electrical transducer signal withinsaid frequency band.